Organic EL elements are broadly classified into low-molecular-weight organic EL (hereinafter abbreviated as OLED) elements and polymer organic EL (hereinafter abbreviated as PLED) elements. With the OLED element, it has been found that a copper phthalocyanine (CUPC) layer is provided as a hole injection layer thereby enabling one to improve initial characteristics, e.g. a lowering of drive voltage, an improved luminous efficiency and the like, and also to improve a life characteristic (see, for example, Non-patent Document 1).
On the other hand, with the PLED element, it has been found that when using polyaniline-type materials (see, for example, Non-patent Documents 2 and 3) and polythiophene-type materials (see, for example, Non-patent Document 4) as a hole transport layer (buffer layer), similar effects are obtained.
It has also been found that when using, at a cathode side, metal oxides (see, for example, Non-patent Document 5), metal halides (see, for example, Non-patent Document 6), and metal complexes (see, for example, Non-patent Document 7) as an electron injection layer, initial characteristics can be improved. These charge injection layer and buffer layer have now been in general use.
In recent years, a charge transporting vanish in the form of an organic solution using a low-molecular-weight oligoaniline-type material has been discovered, and it has now been appreciated that excellent EL element characteristics are shown when inserting a hole injection layer obtained by use of the varnish into an EL element (see, for example, Patent Document 1).
A vacuum deposition material has been widely used as a hole injection material for OLED elements. The problems involved in the vacuum deposition material reside in that they are in an amorphous solid and should have diverse characteristic properties such as sublimability, high heat resistance, and appropriate ionization potential (hereinafter abbreviated as Ip), and thus, limitation is place on the type of material.
Since a vacuum deposition material is difficult to dope, high charge transportability is unlikely to be shown with the film obtained by a vacuum deposition method, with a difficulty in increasing a charge injection efficiency. Moreover, CuPC used as a hole injection material is very irregular in shape and is thus disadvantageous in that when the material is incorporated into other organic layers of an EL element in small amounts, characteristics lower.
Although conjugated oligomers or polymers are materials having high charge transportability, most of them are so low in solubility that it is difficult to prepare a varnish and thus, film formation thereof is possible only by a vacuum deposition method. Particularly, in non-substituted thiophene oligomers, molecules each combined more than four subunits are mostly insoluble in every solvents.
For use as hole transport materials of PLED elements, they should have demand characteristics such as high charge transportability, insolubility in solvents for light-emitting polymer such as toluene, appropriate Ip and the like. Polyaniline-type materials and polythiophene-type materials, which have been frequently employed at present, have the problems in that they contain, as a solvent, water that has the possibility of facilitating element degradation, they are so low in solubility that limitation is placed on the selection of solvent, material coagulation is liable to occur, and limitation is placed on the manner of uniform film formation.
On the other hand, synthesis of compounds having a 1,4-dithiin ring have been reported in document (e.g. Non-patent Documents 8 to 10). The processes of preparing compounds having a 1,4-dithiin ring that are set out in Non-patent Documents 9 and 10 are not only disadvantageous in that those processes have a number of steps and are difficult to mass-produce, but also low in yield, thus improvements thereof being necessary.
Non-Patent Document 1:                Applied Physics Letters, U.S.A., 1996, Vol. 69. pp. 2160-2162        
Non-Patent Document 2:                Nature, Britain, 1992, Vol. 357, pp. 477-479        
Non-Patent Document 3:                Applied Physics Letters, U.S.A., 1994, Vol. 64, pp. 1245-1247        
Non-Patent Document 4:                Applied Physics Letters, U.S.A., 1998, Vol. 72, pp. 2660-2662        
Non-Patent Document 5:                IEEE Transactions on Electron Devices, U.S.A., 1997, Vol. 44, pp. 1245-1248        
Non-Patent Document 6:                Applied Physics Letters, U.S.A., 1997, Vol. 70, pp. 152-154        
Non-Patent Document 7:                Japanese Journal of Applied Physics, 1999, Vol. 38, pp. 1348-1350        
Non-Patent Document 8:                Journal of American Chemical Society, 1953, Vol. 75, pp. 1647-1651        
Non-Patent Document 9:                Heterocycles, 1984, Vol. 22, p. 1527        
Non-Patent Document 10:                Heterocycles, 1987, Vol. 26, pp. 939-942        
Patent Document 1:                Japanese Patent Laid-open No. 2002-151272        